Due to the ability of integration with complementary metal-oxide-semiconductor (CMOS) circuitry, wide bandwidth and highly compact size, the micro-machined capacitive transducers have been widely developed as condenser microphones, accelerometers, pressure sensors and ultrasound devices, etc. However, the performance of these capacitive transducers may be limited by the intrinsic physical laws.
Capacitive transducers with parallel-plate structure most often suffer design trade-offs such as bandwidth and mechanical sensitivity (e.g. for sensors), thermal noise and passive damping (e.g. for sensors), output power and collapse voltage (e.g. for actuators), etc., which result from the nonlinearity of the electrostatic force. Furthermore, the intrinsic properties of the electrostatic force may generate “stiffness softening” effect and results in a collapsed structure when the restoring stiffness force fails to maintain or counter the increasing electrostatic force. The occurrence of collapse for parallel-plate capacitive transducers limits their performance.
For the design trade-off between the transducers' bandwidth and their mechanical sensitivity, in order to expand their bandwidth, the transducers need to be less compliant, which results in the mechanical sensitivity dropdown. Moreover, the compliance of the transducers may also affect the value of restoring force to balance the electrostatic force applied. In the case of capacitive transducers with parallel plate structure, the electrostatic force “softens” the structure of transducers and cause the transducer structures to collapse when the electrostatic force becomes greater than the restoring force that the structures are able to provide. The collapse of the capacitive transducers is then decided by the bias voltage and the gap size between their electrodes. The higher the bias voltage, the transducers may have higher electrical sensitivity but become more likely to collapse. The situation may be worse when designing capacitive actuators. Capacitive actuators generally have a narrower gap size for higher electrostatic energy density, which, however is limited by the ability to avoid a collapse.
Another specification of the capacitive transducers is the mechanical noise floor, generally believed to be determined by their passive damping coefficient. The smaller the damping, the lower the noise floor becomes. However, lightly damped transducers may display deteriorated dynamic performance and results in the failure of the transducers' design.
For a transducer array, the sensitivity tolerance between each transducer cell is critical for the performance of the array. Due to the fabrication issues, there are variations in the mechanical sensitivity of the micro-machined capacitive transducers, which often shows ±1 dB to ±2 dB sensitivity tolerance for the commercial products.
The preceding design trade-offs have become major limitations in improving the capacitive transducers' performance. In addition, conventional transducers employ capacitive tuning where tuning is limited in a single direction, or magnetic tuning which requires a complex transducer structure.